Internally fabricated semiconductor lasers may be used as components in optical transceivers for digital communications products. It may be useful for the laser to operate within as small an electrical power budget as possible, while providing sufficient optical power to span the communication link with low bit-error rate. The efficiency of the laser may therefore be important for the competitiveness of the overall transmitter.
Laser efficiency is in part determined by an effective injection of electrical carriers in the active layer where the Quantum Wells sit that provide light amplification. When the laser is used in high temperature conditions, the injection of electrical carriers into the Quantum Wells is made increasingly difficult as the n-type carriers (electrons) tend to overflow the confinement barrier formed by the p-doped cladding, and then diffuse into the p-doped cladding where they recombine, in a non-radiative way, with majority incoming p-type carriers, instead of being involved in radiative recombination within the Quantum Wells that generate photons.
In the Indium-Gallium-Aluminum-Arsenide/Indium-Phosphide (InGaAlAs/InP) semiconductor material system, which is generally used to make 1310 nanometer (nm) emitting lasers when high temperature is a constraint, the electrical confinement of n-carriers may be improved by inserting a thin layer of lattice-matched Indium Aluminum Arsenide (InAlAs) between the Multiple-Quantum-Wells (MQWs) and the InP p-type cladding. This is because the Band-Gap energy of InAlAs is larger than the band-gap energy of InP, and therefore provides a higher Conduction Band Energy Discontinuity (Delta Ec) that prevents the n-type carriers from migrating to the p-doped InP cladding.
It is noted that the term “lattice matched” refers to matching of lattice structures between two different semiconductor materials. This allows a region of band gap change to be formed in a material without introducing a change in crystal structure, and is what allows the construction of advanced light-emitting diodes and diode lasers. Lattice matched structures have the same lattice constant, or lattice parameter, which refers to the physical dimension of unit cells in a crystal lattice. Thus, the lattice constant is the distance between atoms in cubic-cell crystals, and may be used as a measure of structural compatibility of various crystals. For example, gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), and aluminum arsenide (AlAs) have almost equal lattice constants, making it possible to grow almost arbitrarily thick layers of one on the other one. Though sometimes effective, the lattice matched InAlAs layer does not always prevent n-carriers from spilling into the p-doped InP cladding, particularly when the temperature is higher than 80° C. and the wavelength emission is lower than 1270 nm.